At present, microprocessors are providing on-chip clock frequencies on the order of 1 GHz. The International Technology Roadmap for Semiconductors (ITRS) forecasts on-chip clock rates of about 10 GHz in 2011. Performance in the GHz range can only be fully exploited if the off-chip interconnection technology provides appropriate bandwidth, a constraint that will become increasingly more severe during the next ten years. This means that the challenge of routing signals off-chip and into the system in the GHz frequency range is expected to exceed that of achieving on-chip performance at these frequencies. In addition, highly parallel, next-generation computational systems will require highly dense connection networks containing many long-distance connections. In such highly connected, highly parallel systems, the module-to-module and long distance chip-to-chip connections are responsible for the majority of the power dissipation, time delay and surface area. Thus, it has become critically important to minimize the area, power and time delay of the chip-to-chip and module-to-module interconnects while, at the same time, increasing density and bandwidth. The Semiconductor Industry Association (SIA) has recognized these challenges and identifies interconnects as the primary chip-related technology with the largest potential technology gaps.
By replacing electrical intramodule and module-to-module connections with optical communication links, the communication bottleneck can be relieved. In recent years considerable R&D effort has been devoted to developing optical chip-to-chip interconnects to reduce this microelectronics interconnection problem. It has now been demonstrated that optical interconnects have the potential to increase communication speed and reduce the volume, crosstalk and power dissipation of the connections.
Despite recent progress and the demonstrated potential of optical interconnects this technology is still at an early stage and practical realization of its potential will require more efficient approaches and further improvements in performance. In particular, there is a need for reconfigurable interconnect technologies capable of higher density of connections while reducing power, area and cost.
Modern electronic circuits consist of semiconductor chips mounted on circuit boards. These boards are in turn assembled into modules and eventually into cabinets or chassis. There are:
Very short interconnectionson the chipShort interconnectionschip to chipMedium interconnectionsboard to boardLong interconnectionsmodule or chassis level
These interconnections are formed electrically with metallic (usually aluminum or copper) pathways (traces). The density and total number of these interconnections has become a major challenge due to physical space requirements and to interfering cross talk.
It is clear, that an optical processing format could help significantly with these interconnections, but such help can not be applied until appropriate electro optic interfaces are available. Relatively simple optical connections are being implemented via waveguides in boards or free space links. These are limited in the number of practical connections that can be implemented. Sophisticated, all optical, arrays are being developed, which in general require several cubic mm of space and multiple light sources. Only the larger and most sophisticated can be reconfigurable.
Optical information processing can perform a multitude of operations because of its complex amplitude processing capability using several dimensions. There are two approaches to such processing, which have been pursued.    1. Approach #1 uses the Fourier-domain.    2. Approach #2 uses the Spatial-domain.
Both approaches have trade-offs, and difficulties with signal to noise, which have brought forward many versions of distortion invariant matched filters. It has been demonstrated though that optics can offer advantages of optical parallelism, high resolution, and massive connectivity.
However, the prevalent electronic digital computing offers much greater flexibility for various implementations, and will not soon give way to an all-optical approach. An electro-optic interface, which would allow a hybrid electronic-digital/optical approach, could solve two problems with emerging processors.
Problem 1.
As integrated circuits (and modules) have grown in complexity and size, the number of in-out interconnections has become almost impossible to implement. The fundamental problem is that there is simply no surface area available for more electrical conductors. An optical interconnection scheme could help immeasurably because no conductive traces are required, fewer connections are required (does not require signal plus ground) and additional dimensions can be utilized. There is no longer a need to remain planar.
Problem 2.
As increasingly dense systems of higher speeds are created, there is a need for parallel processing, which requires “long distance” interconnects. There is an adverse effect on power, speed, and heat. Although desirable for some emerging systems, it is extremely difficult to make the parallelism dynamically reconfigurable, and crosstalk can be a problem, especially if any analog signals are present. An optical interconnection scheme could help significantly because it removes the requirement for long traces, can be reconfigurable, allows high speed, results in less power density, and minimizes cross talk.
An electro-optical interconnection requires that some form of modulation be implemented whereby either the electrical signal modulates an optical signal, or the optical signal modulates the electrical signal, or both. This is likely to be of the Spatial-domain type since most Fourier type are optical to optical. Spatial modulators have been made using special crystals (for example Lithium Niobate, Barium Titanate), liquid crystals, and various polymers. Special crystals are prohibitively expensive and cumbersome to work with, but can be fast (GHz). Liquid crystals are slow. Polymers are attractive because they offer low cost, ease of fabrication, and high speed. Polymer development work is ongoing at University type laboratories, and some telecommunication companies. To date, these materials suffer from drive level and lifetime problems. These problems could be alleviated, if a separate enhancement to the modulation mechanism within the polymer were provided. Required characteristics of the polymer would be made less stringent.